In recent years, extensive studies have been made of techniques of crystallizing an amorphous semiconductor film or improving the crystallinity of a crystalline semiconductor film (i.e., a non-single-crystal semiconductor film that is polycrystalline, microcrystalline or of like crystallinity) formed on an insulating substrate such as a glass substrate by subjecting it to laser annealing. A typical example of such a semiconductor film is a silicon film.
Glass substrates are advantageous over quartz substrates that have widely been used conventionally in that they are inexpensive and high in workability and can easily provide a large-area substrate. This is the reason why the above studied have been made. The reason why lasers are used favorably in crystallization is that glass substrates have low melting points. Lasers can apply high energy to only a non-single-crystal film without changing the temperature of a substrate to a large extent.
Having high mobility, crystalline silicon films that are formed by laser annealing are widely used in monolithic liquid crystal electro-optical devices and the like in which pixel-driving TFTs (thin-film transistors) and driver circuit TFTs, for instance, are formed on a single glass substrate by using such a crystalline silicon film. Since a crystalline silicon film formed by laser annealing is constituted of a number of crystal grains, it is called a polysilicon film or a polycrystalline semiconductor film.
A laser annealing method in which a pulse laser beam emitted from a large output power excimer laser or the like is processed by an optical system so as to be shaped into a several centimeters square spot or a linear beam of several millimeters in width and tens of centimeters in length and scanning is made with the processed laser beam (i.e., a laser beam illumination position is moved relative to an illumination surface) is favorably used because it provides high mass-productivity and is superior from the industrial point of view.
In particular, in contrast to the case of using a spot-like laser beam that requires scanning in two orthogonal directions, the use of a linear laser beam can provide high mass-productivity because the entire illumination subject surface can be illuminated with laser light by scanning it with the linear laser beam only in the direction perpendicular to its length direction. The scanning is made in the direction perpendicular to the length direction because the scanning in such a direction is most efficient. Because of the high mass-productivity, the use of a linear laser beam is now becoming the mainstream in the laser annealing.
There are several problems in the case of performing laser annealing on a non-single crystal semiconductor film by scanning it with a pulse laser beam that has been processed into a linear shape. Among those problems, one of the particularly serious problems is that laser annealing is not performed uniformly over the entire film surface. When linear laser beams started to be used, there occurred a marked phenomenon that stripes were formed at beam overlapping portions. A film showed much different electrical characteristics from one stripe to another.
FIG. 1A shows how such stripes are formed. Stripes appear depending on the manner of light reflection when the surface of a laser-annealed silicon film is observed.
FIG. 1A is of a case where a linear laser beam extending in the right-left direction in the paper surface that is emitted from a XeCl excimer laser is applied while scanning is made in the top-to-bottom direction in the paper surface. It is understood that the stripes of FIG. 1A originate from the manner of overlapping of shots of pulse laser beams.
Where an active matrix liquid crystal display was manufactured by using a silicon film that exhibits stripes as shown in FIG. 1A, there occurred a problem that similar stripes appeared on the screen. This problem is now being solved by improving a non-single crystal semiconductor film as a subject of laser beam illumination and making the linear laser beam scanning pitch (interval between adjacent linear laser beams) finer.
As the above type of stripes become less conspicuous, non-uniformity in the energy profile of a beam itself comes to appear.
In general, in forming a linear laser beam, an original rectangular beam is processed into a linear shape by inputting it to a proper optical system. A rectangular beam having an aspect ratio of 2 to 5 is modified into a linear beam having an aspect ratio of 100 or more by an optical system of FIG. 2, for instance. This optical system is so designed that the intrabeam energy profile is uniformized at the same time.
The apparatus of FIG. 2 has a function of converting a laser beam emitted from an oscillator 201 (approximately rectangular at this stage) into a linear beam with an optical system denoted by reference numerals 202-204, 206, and 208 and applying the linear beam. Reference numerals 205 and 207 denote a slit and a mirror, respectively.
A cylindrical lens group (also called a multiple cylindrical lens) 202 has a function of dividing a beam into many beams. The many divided beams are combined by a cylindrical lens 206.
The above components are needed to improve the intrabeam intensity profile. A combination of a cylindrical lens group 203 and a cylindrical lens 204 has the same function as the combination of the cylindrical lens group 202 and the cylindrical lens 206.
That is, the combination of the cylindrical lens group 202 and the cylindrical lens 206 has a function of improving the intensity profile of a linear laser beam in the longitudinal direction and the combination of the cylindrical lens group 203 and the cylindrical lens 204 has a function of improving the intensity profile of a linear laser beam in the width direction.
An optical system having a role of uniformizing the intrabeam energy profile is called a beam homogenizer. The optical system of FIG. 2 is also a beam homogenizer. One method of uniformizing the energy profile is to divide an original rectangular beam, enlarging the divided beams, and then combining the enlarged beams.
It appears that a beam that has been reconstructed after dividing an original beam in the above manner would have a higher degree of uniformity in energy profile as the beam division is made finer. However, when a beam obtained in the above manner was actually applied to a semiconductor film, stripes as shown in FIG. 1B occurred in the film in spite of fine beam division.
Like the case of FIG. 1A, FIG. 1B is of a case where a linear laser beam extending in the right-left direction in the paper surface that was emitted from a XeCl excimer laser was applied to a silicon film while scanning was made in the top-to-bottom direction in the paper surface. However, in the case of FIG. 1B, the scanning conditions were set properly so that no marked stripes as shown in FIG. 1A appear.
As shown in FIG. 1B, innumerable stripes are formed perpendicularly to the longitudinal direction of a linear laser beam. Stripes of this type should be formed due to a striped energy profile of an original rectangular beam or the optical system.
To investigate whether stripes were caused by a striped energy profile of an original rectangular beam or the optical system, the inventors conducted a simple experiment. In the experiment, it was studied how stripes varied as a rectangular laser beam was rotated before entering the optical system.
No variation was observed at all. Therefore, it was confirmed that the optical system caused stripes rather than an original rectangular beam. It is concluded that stripes are an interference fringe of light because the optical system uniformizes the energy profile of a single-wavelength, phase-equalized beam (a laser beam is phase-equalized because a laser produces high-intensity light by equalizing the phase) by dividing it and combining the divided beams.
FIG. 3 illustrates a light interference fringe 302 in a linear laser beam 301 that is formed by the optical system of FIG. 2. In FIG. 3, symbol I represents the laser light intensity. The interference fringe 302 is produced in such a manner that when beams obtained by dividing an original beam by the cylindrical lens groups 202 and 203 of the optical system of FIG. 2 are combined by the cylindrical lenses 204 and 206, the divided beams interfere with each other and a stationary wave is thereby formed in the beam.
That is, the reason why sharp, periodic interference peaks are generated is that divided beams are superimposed one on another in the same region on an illumination surface.
As shown in FIG. 3, the amplitude of waves varies periodically. In the case of the optical system of FIG. 2, three waves are formed per one period in the longitudinal direction of a linear beam.
The number n of waves (i.e., the number of interference peaks) per pitch and the number s of lenses of the cylindrical lens group 202 satisfy the following relationship: EQU n=(s-1)/2 (s: odd number) EQU n=s/2 (s: even number)
In the optical system of FIG. 2, the number n is equal to 3 because the number s of lenses of the cylindrical lens group 202 is 7 (odd number).
In this case, an interference state shown in FIG. 4A is obtained. FIG. 4A, which was obtained by a computer calculation, shows an interference state in a linear laser beam at a certain time point. The horizontal axis of FIG. 4A corresponds to the position in the longitudinal direction of a linear laser beam, and the square of a value on the vertical axis of FIG. 4A corresponds to the light intensity in an actual interference state. For example, the interference state of FIG. 4A is actually observed as the light intensity profile shown in FIG. 3.
Where the number s of lenses of the cylindrical lens group 202 is equal to 8, an interference pattern as shown in FIG. 4B is obtained.
In FIGS. 4A and 4B, the square of the amplitude represents the strength of interference (i.e., the degree of an action that beams having the same phase intensify each other) and parameter d is defined as the pitch of interference peaks.
The curves of FIGS. 4A and 4B were obtained by a computer simulation and actual interference fringes of laser beams do not exhibit so clear strong and weak portions. This is considered due to dispersion, refraction, and loss of light that are caused by slight deviations in the optical system, the materials of the components of the optical system, and processing errors of the optical system, energy dispersion in a semiconductor film that is caused by heat conduction, and other factors.